1. Field of the Invention
The present invention relates to a probe for detecting the movement of a magnetic element, the probe having a plurality of output channels.
2. Discussion of Prior Art
The use of a magnetic sensor in cooperation with one or more projections on a shaft to give an output from which shaft rotational speed may be determined is well known. In a sensor of this type a coil forming part of an electric circuit is wound around a magnetically energised pole piece. A voltage is induced in the coil by changes in the magnetic flux pattern associated with the pole piece, caused by the relative movement of a projection made of magnetic material in the proximity of the pole piece. The resulting voltage signal can be analysed to determine the rotational speed of the shaft.
U.S. Pat. No. 7,148,679, the contents of which are incorporated herein by reference, discloses a magnetic sensor of this type, which includes a transformer coupled to the coil for detecting the induced voltage. The use of a transformer allows the primary circuit to have one or a few turns on the pole piece. These turns generate a low voltage signal, which is transformed to a high output voltage at the secondary coil of the transformer.
In some circumstances it is desirable to provide sensor redundancy. One such circumstance is when using a sensor to accurately measure the engine speed of a gas turbine engine in an aircraft. Almost all aircraft components are provided in duplicate or triplicate to provide backup in the event of a component failure. For a measurement as important as engine speed it is essential to have a backup.
FIG. 1 illustrates a magnetic sensor having three output channels A, B, C. Each output channel is provided as a primary winding 10 around a common magnetic pole piece 11. The pole piece is coupled to a permanent magnet 12. Each winding is part of a separate circuit. A toothed wheel 13, called a phonic wheel, is located on the engine drive shaft (not shown), or is coupled to it, and rotates as the engine shaft rotates. The pole piece 11 is located close to the rotating phonic wheel 13. The teeth on the phonic wheel are formed from a magnetic material i.e. a material with a magnetic permeability substantially greater than air, and are typically formed from a ferromagnetic material. Movement of the phonic wheel relative to the pole piece results in a change in the magnetic flux experienced by each of the windings, which in turn induces the voltage in each of the output circuits. The induced voltage is detected as a periodic signal in a secondary transformer circuit 14, from which the speed of the wheel can be determined.
The problem with this design is that each of the output channels provides a different magnitude of signal as a result of the different physical positions of the primary windings around the pole piece. Each of the windings will experience a different rate of change of magnetic flux as a result of the movement of the magnetic elements on the phonic wheel. There is a higher degree of magnetic flux change closer to the phonic wheel and therefore the primary coil positioned at the front of the pole piece produces a higher output compared to those positioned further back. It is important that each channel receives the same signal magnitude, as, when a channel is switched because of a coil failure, the electronics should see the same minimum signal amplitude. If the new signal amplitude is much smaller it might not be detected. One solution would be to design the electronics for the weakest signal, but in this case the strongest signal might be too strong at the highest speed and would have to be attenuated.
In the past, the outputs have been balanced by varying the number of turns on the primary coils. But this is not an easy thing to do, as there are a relatively low number of turns (typically one to four) to adjust. Accordingly, it sometimes requires control of the number of turns down to a half turn and additionally accurate positioning of the start and finish of the primary coil on the pole piece. This method of matching the output is technically difficult, time consuming and not always successful.